The parasitic nematode Angiostrongylus cantonensis or the rat lungworm is a parasitic worm residing in the pulmonary arteries of rats. The parasite stays alive in the feces of infected rats.
When snails and slugs ingest the larvae, the parasite matures, but they do not grow into adult worms.
When rats eat the infected snails or slugs, the larvae grow further in the rats and mature to adult worms.
Humans are prone to infection when they consume food or water infected by the parasite, or while consuming uncooked snails or slugs that are infected by the larvae.
While the larvae do not mature in humans, they can create serious complications affecting the central nervous system and, in rare cases, even death.
Definitive Host-Intermediate Host-Accidental Host
The male worm of the species, Angiostrongylus cantonensis, is 15.9–19.0 mm long, while females are longer and grow up to 21–25 mm in length.
The requirement for the species is that the definitive host should be a terrestrial mammal; the intermediate host should be an invertebrate; and the transport hosts are vertebrate or invertebrate.
Definitive hosts are those where the worms grow and reproduce and reach the adult stage. Rats and rodents are definitive hosts as the species are usually from the genus Rattus, especially from Rattus norvegicus and Rattus rattus.
Snails, slugs, and semi-slugs act as intermediate hosts, which is essential for the development of the larvae to the next stage and the life cycle of the species is continued.
Considered the most invasive of species, the African giant land snail is consumed typically raw in many regions, as it is considered a delicacy.
Although native to Africa, the species lives in almost all countries.
In general, each snail lays 10–400 eggs and in favorable conditions this increases to 300–1000 eggs per year. The time span for hatching the egg is 11 days under humid climate and the estimated lifetime is between 3 and 5 years; however, it may live up to 9 years.
Due to the large deposit of eggs in a short span of time, an infected snail can contain thousands of larvae that can be spread across easily.
There are hosts in which the parasite does not progress to the next stage but help the species to move - these are called accidental hosts. Frogs, toads, crabs, and shrimp act as accidental host for the species.
When humans consume the raw or uncooked intermediate host or accidental host or consume food and water infected by the species, the larvae reach the human brain.
In primates, the species does not grow further, but causes symptoms and diseases such as eosinophilic meningitis and eventually dies.
Life Cycle of Rat Lungworm
The first stage of the larvae seen in the feces of definitive hosts—rats—is only 0.27–0.30 mm in length. When the snails come into contact with the feces, the larvae can penetrate the body of the snail or enter through the respiratory pore.
The snails or slugs ingest the infected feces and become intermediate hosts. The larvae will grow to stage 2, to about 0.42–0.47 mm in length and further to stage 3 to about 0.42–0.49 mm in length in the snails or slugs. The life cycle of the larvae ends at this stage if the snail dies or if it is consumed by humans.
Instead of humans, if rats consume the infected snails, the third stage larvae enter the bloodstream of the rat by penetrating the intestine walls. Some larvae enter the central nervous system and develop further in the brain. The stage 4 larva is 0.85–1.00 mm long.
From the brain, larvae pass to the venous circulatory system and reach the heart’s right ventricle and then to the pulmonary arteries, where they mature, lay eggs, and hatch into first stage larvae in the lung tissues of the rat.
The worms molt twice before hatching the eggs and twice again before they reach adult stage.
Breaking into the respiratory tract, the first stage larvae move into the trachea. They are swallowed and released in the feces. The entire cycle from the ingestion of third stage larvae in the rats to the releasing of first stage larvae in the feces is completed in 45 days.
Spread of Disease
Lack of proper sanitation increases the number of definitive hosts such as rats and rodents. Rainforest, lakes, ponds, and other moist environments such as gardens favor the growth of intermediate hosts such as snails, slugs, and semi-slugs.
Consumption of raw produce susceptible for infection, habits of eating uncooked food such as snails, toads, and crabs (could be intermediate and accidental hosts), using catchment water for household purposes such as brushing, bathing, or washing vegetable and fruits that could have been contaminated are some causes for the spread of the disease.
Though completely eradicating rats or snails is not possible, the risk of infection can be reduced by adopting suitable preventive methods.
Further Reading